The present invention relates in general to integrated circuits, and in particular to improved method and circuitry for implementing a voltage-controlled oscillator (VCO).
A voltage-controlled oscillator is an oscillator that generates an output signal with a frequency that varies in response to an input control voltage. VCOs can be found in many circuit applications. For example, a VCO is the central block in phase-locked loops (PLLs) and as such is one of the most important blocks in data commination circuitry. Many circuit applications demand a wide output frequency range for the VCO. Because of its non-linear response to the input control voltage, a VCO typically requires a large gain, Kvco, if a wide output frequency range is desired. The VCO gain, however, has the undesirable effect of amplifying any noise in the input voltage signal to the VCO, resulting in what is commonly referred to as phase noise. This phase noise in turn contributes to output signal jitter and adversely affects the overall performance of the PLL. While reducing Kvco helps reduce the phase noise, it limits the frequency range of the VCO and limits the circuit""s ability to compensate for process and environment variations.
To address both of these requirements, i.e., large gain (wide frequency range) and low phase noise, VCOs have been implemented that can switch between multiple narrow frequency bands. The VCO operates in each narrow band of frequency with lower gain and therefore lower phase noise, while the multiple bands combined provide for the wider overall frequency range. Moreover, since inductor/capacitor (LC) based VCOs have lower phase noise than traditional VCOs using ring oscillators, they are being used more and more in low noise applications. In this type of VCO, the oscillation frequency F is given by the inductance L and capacitance C as follow:
xe2x80x83F=1/(2xc3x97pixc3x97sqrt(Lxc3x97C))
In the LC based VCO the phase noise is directly related to the amount of parasitic resistance in series with L or C. FIG. 1 shows a conventional LC VCO 100 with band switching. VCO 100 includes resonant LC tank circuit 102 having an inductor 1L coupled to a variable capacitor (or varactor) 1C. Multiple switchable capacitors 1C(1) to 1C(n) connect to tank circuit 102 as shown. Switches 1S(1) to 1S(n) are digitally controlled and when activated modify the total capacitance in the LC circuit causing the VCO frequency to switch frequency bands. A similar band switching can be achieved by switching additional inductors in parallel (or in series) with the active circuit.
A drawback of this approach, however, is the finite resistance that is introduced by the switches. Switches 1S(1) to 1S(n) are typically implemented by metal-oxide-semiconductor (MOS) transistors. When turned on, the on-resistance of the MOS switch is placed in series with its respective switchable capacitor. This parasitic resistance adds to the loss in the tank circuit and deteriorates the overall VCO performance. A potential problem also arises when the switches are in their off state. Under this condition, the switched node of the capacitor is left floating and as such is made susceptible to noise.
There is therefore a need for improved implementations of VCOs with band switching capability.
The present invention provides method and circuitry for implementing VCOs with improved frequency band switching. Broadly, the VCO of the present invention uses differentially-coupled varactors to implement the different frequency bands. According to a specific embodiment, one side of a varactor couples to the tank circuit and the other side is coupled either to ground or to the positive power supply VDD without introducing any series parasitic resistance. When switched to ground, the VCO tank sees the full capacitance of the varactor. When switched to VDD, the capacitance of the varactor as seen by the tank circuit is minimized. In one exemplary embodiment, the present invention implements the varactors using a field effect transistor structure with the gate terminal acting as one node of the varactor and the source/drain terminals coupled together providing the other node of the varactor.
Accordingly, in one embodiment, the present invention provides a voltage controlled oscillator (VCO) including: a differential inductor/capacitor (L/C) tank circuit coupled between a first node and a second node; and at least one pair of varactors coupled to the tank circuit, the pair of varactors including a first varactor connected between the first node and a switching node, and a second varactor connected between the second node and the switching node, wherein, an output frequency band of the VCO is modified by switching the potential at the switching node.
In a more specific embodiment of the present invention the VCO further includes a switching circuit coupled to the switching node and configured to switch the potential at the switching node between a high voltage and a low voltage. The switching circuit may be an inverter with the high voltage being the circuit power supply voltage and the low voltage being ground.
In another embodiment, the present invention provides a differential voltage controlled oscillator including: an inductor coupled between a first node and a second node; a cross-coupled pair of p-channel transistors respectively coupling the first and second nodes to a power supply node; a cross-coupled pair of n-channel transistor respectively coupling the first and second nodes to ground; a first fixed varactor having a first terminal connected to the first node and a second terminal connected to receive a control signal; a second fixed varactor having a first terminal connected to the second node and a second terminal connected to receive the control signal; a first variable varactor having a first terminal connected to the first node and a second terminal connected to a switching node; a second variable varactor having a first terminal connected to the second node and a second terminal connected to the switching node, and an inverter having an input coupled to receive a switching signal and an output connected to the switching node.
In yet another embodiment, the present invention provides a method for switching a frequency band of a voltage controlled oscillator output signal, the method including: adding a first capacitance to a differential tank circuit by applying a first signal to a common node of a differential pair of varactors; and adding a second capacitance to the differential tank circuit by applying a second signal to the common node of the differential pair of varactors. The common node of the differential pair of varactors is an ac ground.
The detailed description and the accompanying drawings provide a better understanding of the nature and advantages of the signal level converter of the present invention.